1. Field of the Invention
This invention relates to processes for the preparation of extended residual, controlled release sulfur coated fertilizers and to products produced thereby.
2. Description of the Related Art
Controlled or slow release fertilizers of the general type contemplated herein and methods for the production of such fertilizer products are disclosed in copending parent U.S. patent application Ser. No. 07/666,339, filed Mar. 8, 1991. Specifically, the extended residual, controlled release sulfur coated fertilizer products provided in the copending application release their fertilizer core material such as urea from their coatings at a diffusion controlled rate in an essentially linear release pattern over time.
As disclosed in application Ser. No. 07/666,339, sulfur coated fertilizer products are produced by applying appropriate polymer topcoats over fresh primary sulfur coatings which have sufficient amorphous sulfur content at the time the polymeric topcoat is applied thereover to preserve the surface integrity of the coating with essentially no stress induced discontinuities therein. More particularly, products produced with soluble particulate fertilizer cores including granules and prills coated with a primary sulfur coating and subsequently coated, while the sulfur coating is still fresh, with a polymeric topcoat having particular characteristics, have been found to achieve unique nutrient release patterns and agronomic results compared to those exhibited by prior sulfur coated fertilizers even coated with polymeric topcoats.
The nutrient release characteristics provided in the products of the pending application were achieved by diffusion kinetics similar to the release pattern shown by expensive polymer coated fertilizers requiring high coating levels and very precise application equipment and techniques of production. The products described therein are prepared at relatively low cost with substantially reduced polymer coating levels and with comparatively simpler process equipment utilizing simpler production techniques.
Thus, the products disclosed in the copending application possess exceptional functional qualities and the processes for their production have been found to be effective. Nonetheless, it has been a continuing problem to reduce the manufacturing tolerances inherent in the production processes provided therein and yet produce acceptable extended residual, controlled release sulfur coated fertilizers having outer polymer coatings thereover. For example, as previously taught, it has been considered essential to employ only a fresh sulfur coated fertilizer core as the base layer over which the polymeric layer is applied. That is, in order to prevent excessive embrittlement or cracking of the sulfur layer which would adversely impact the release characteristics of the end product to the extent that the resulting product would be unacceptable for use, it has been believed to be necessary for the fresh sulfur layer to contain a content of carbon disulfide insoluble amorphous sulfur above a critical lower limit at the time at which the polymer coating is applied thereover. This processing limitation regarding use of fresh sulfur having the requisite amorphous sulfur content has placed time constraints on the manufacturing process that producers would prefer to avoid under certain circumstances such as are encountered in batch processing manufacturing facilities in order to enable more efficient process scheduling and more cost effective operations.